1. Field of the Invention
This invention relates to a process of biotreating wastewater from pulping industries to remove or significantly reduce sulfides.
2. Description of the Prior Art
Natural fibers play an essential role in the operation of innumerable industries. These industries include, but are not limited to, wood pulp bleaching facilities, general pulp processing facilities and natural fiber processing facilities. Developing said natural fibers, in order to achieve a stage of marketability, usually requires a process (referred to as a pulping process), in which water is utilized as a transporting medium.
During the pulping process, naturally existing sulfur compounds, such as sulfides, are released into the water. (Mercaptans have comparable characteristics to sulfides and, unless otherwise specified, the statements made hereafter in reference to sulfides would be applicable to mercaptans as well.) While the sulfide-containing wastewater is sent to or is in a biological treatment system, some sulfides are released into the atmosphere. The concentration of numerous sulfidic species, in the atmosphere, above and below their odor threshold is toxic to human and to animal life. The presence of sulfides in wastewater has other adverse consequences as well. One such adverse consequence is the development of a chemical oxygen demand (COD), resulting in oxygen depletion in the receiving water after discharge of the wastewater and, thus, leading to environmental pollution and/or levies, toxic effects and serious stench. The odor is extremely objectionable and is related to the chemical characteristics of sulfides. Further, release of the sulfur-containing effluent water streams of the pulping industry into the environment endangers aquatic life. Potential regulatory changes may result in more stringent controls and increased costs and may require the pulp and paper industry to substantially reduce the constituents in its effluent streams.
Ideas for treatment of the wastewater from the pulping processes have been proposed. However, said ideas have been neither aimed at nor capable of removing sulfides from wastewater streams of pulping processes.
In determining economical and efficient solutions for the removal of sulfur-containing compounds, several factors should be considered. Generally, two types of methods are available for the removal of sulfur-containing compounds: physicochemical treatments and biological treatments. Physicochemical treatments (including peroxygens-caustics electrodialysis and reverse osmosis) are expensive and produce large streams of wastewater. Wastewater treatment processes for the removal of sulfides, biological oxygen demand (BOD), COD, total Kjeldahl nitrogen (TKN) and ammonia by utilizing bacteria are generally known. The wastewater could be treated under either aerobic (including anoxic) or anaerobic conditions. (In order to facilitate reference, an aerobic or aerated condition or system would include an anoxic case as well unless otherwise specified.) To remove BOD, COD, TKN and/or ammonia, many municipal and industrial facilities apply bacteria either in a single-pass aerated treatment system utilizing tanks, ponds or film reactors or in a multi-step process performed in an aerated activated sludge system.
Several patents have been issued, mostly in the past decade, that are focused on biotreatment of wastewater.
Sublette, U.S. Pat. No. 5,480,550, issued on Jan. 2, 1996, patents a biotreatment process of caustic waste streams containing inorganic sulfides to effect neutralization of the caustic and oxidation of sulfides to sulfate. The process is based on the contact of the caustic streams with mixed, flocculated cultures of a sulfide-oxidizing bacterium from the genus Thiobacillus and various heterotrophs. Being premised on the treatment of caustic waste streams that only contain inorganic sulfides, this process considers neither wastewater of lower pH nor the removal of organic sulfides nor streams containing both organic and inorganic sulfides.
Castaldi et al., U.S. Pat. No. 4,790,940, registered on Dec. 13, 1988, and U.S. Pat. No. 4,737,289, registered on Apr. 12, 1988, patent a process for treatment of wastewater with a combination of Thiobacillus and denitrifying bacteria. The Castaldi patents essentially claim: treating wastewater containing free cyanide to destroy the free cyanide content by treating the wastewater with polysulfide under alkaline conditions, adjusting the pH and treating the resulting wastewater with a treating agent consisting essentially of cultures of bacteria of the genus Thiobacillus in combination with denitrifying bacteria.
Hata, U.S. Pat. No. 4,605,502, registered on Aug. 12, 1986, patents methods for purifying water or water-containing material using microorganisms and living bacterial preprarations as well as method for preparing and storing same. The method comprises contacting the water or water-containing material with a bacterial composition comprising one or more bacterial strains.
The above-listed patents and many other similar inventions have been developed, some of which still exist in the market. Although many different issues have been solved by previously- and presently-existing biotreatment processes, no biotreatment process has strived towards removal of potentially high concentrations of sulfides from the effluent water streams from pulping processes.
The major differences between the present invention and existing patents are in the steps of the process and in the sources of the number of strains of bacteria. In Sublette (U.S. Pat. No. 5,480,550), an autotrophic bacteria is first immobilized by co-culture with at least one floc-forming heterotroph. Then, the co-cultures of the number of strains of bacteria with heterotrophs are suspended in a mineral salt medium sufficient to support growth of the autotrophic bacteria. Finally, the aqueous caustic solution is added to the suspension. In the present invention and in Sublette, co-cultures of a number of strains of bacteria with heterotrophs are used. However, in the present invention, a number of strains of bacteria and co-cultures of a number of strains of bacteria with heterotrophs are simultaneously and additionally applied to wastewater. In Sublette, first, the co-cultures of the number of strains of bacteria with heterotrophs are suspended in a mineral salt medium sufficient to support growth of the autotrophic bacteria. Then, the aqueous caustic solution (wastewater) is added to the suspension. Therefore, in the present invention, as a first source of bacteria, a number of strains of bacteria and, as a second source of bacteria, co-cultures of a number of strains of bacteria with heterotrophs are added simultaneously to the wastewater, while in Sublette wastewater is added to co-cultures of bacteria that are in a mineral salt suspension that allows growth of bacteria. The final product may be wastewater including a number of strains of bacteria and co-cultures of a number of strains of bacteria. However, Sublette and the present invention provide different steps in obtaining that final product.
In Hata, the bacterial composition that is added to the wastewater solely comprises one or more bacterial strains, but does not include any co-cultures of the bacteria with heterotrophs. Despite the quantity of the bacteria disclosed in Hata, no co-culturing of the bacteria with the heterotrophs is disclosed. Hata basically only claims application of bacteria to water.
In Castaldi et al., the only source of bacteria is the treating agent that consists essentially of cultures of bacteria of the genus Thiobacillus in combination with denitrifying bacteria. On the other hand, if the treating agent in Castaldi et al. is considered as a source of bacteria, then there is no co-cultural blend that is being simultaneously applied to the wastewater in Castaldi et al. Therefore, in Castaldi et al., there is only one source of bacteria, and the final product is denitrification, via one application of one treating agent. In the present invention, there are two simultaneous sources via which a number of strains of bacteria are added individually and simultaneously.